1 2019 Tampa Bay Water Quality Assessment

1.1 Background

Light availability to seagrass is the guiding paradigm for TBEP’s Nitrogen Management Strategy. Because excessive nitrogen loads to the bay generally lead to increased algae blooms (higher chlorophyll-a levels) (Figure 1.1) and reduce light penetration to seagrass, an evaluation method was developed to assess whether load reduction strategies are achieving desired water quality results (i.e. reduced chlorophyll-a concentrations and increased water clarity).

Figure 1.1: Guiding paradigm for Tampa Bay seagrass restoration through the management of nitrogen loads.

1.2 Decision Support Approach

Year to year algae abundance (measured as chlorophyll-a concentrations) and visible light penetration through the water column (depth of secchi disk visibility) have been identified as critical water quality indicators in Tampa Bay. Tracking the attainment of bay segment specific targets for these indicators provides the framework for developing and initiating bay management actions. TBEP management actions adopted in response to the annually-assessed decision support results are shown in Figure 1.2.

Figure 1.2: Management actions assigned to each bay segment.

1.3 2019 Decision Matrix Results

Water quality (chlorophyll-a and light penetration) remained supportive of seagrass in Hillsborough Bay (HB), Middle Tampa Bay (MTB), and Lower Tampa Bay (LTB)(Table 1.1, Figure 1.3). The nuisance alga, Pyrodinium bahamense, was again reported in Old Tampa Bay (OTB) during the Summer and Fall 2018, contributing to a small magnitude chlorophyll-a exceedance. In all bay segments, separate algal bloom events contributed to individual stations exceeding the bay segment chlorophyll-a targets (Figures 1.4, 1.5). However, effective light penetration was supportive of seagrass in all bay segments (Table 1).

Figure 1.3: Historic chlorophyll-a annual averages for the four bay segments.

Table 1.1: Results for 2019

Figure 1.4: Decision matrix results from 1975 to 2019.

Figure 1.5: Chlorophyll attainment outcomes by site for 2019.

2 Progress Towards Meeting Regulatory Goals

2.1 Maintaining Reasonable Assurance & TMDL Compliance

In November 2017, the Florida Department of Environmental Protection (FDEP) accepted the 2017 Reasonable Assurance Update (2017 RA Update) as submitted by TBEP in partnership with the Tampa Bay Nitrogen Management Consortium. FDEP concluded that the RA Update demonstrated both attainment of seagrass targets and total nitrogen numeric criteria for 2012-2016. During 2019, all bay segments, excluding Old Tampa Bay, were in compliance with the FDEP regulatory criteria for chlorophyll-a concentrations (Figure 2.1). The second compliance report for the 2017-2021 period will be submitted by March 2019.

Figure 2.1: Attainment of bay segments for chlorophyll criteria from 1975 to 2019.

2.2 2019 Chl-a Monthly Variation Compared to 1974-2018

Chlorophyll-a concentrations were evaluated within the bay on a monthly basis during 2019 and compared to prior years’ levels (Figure 2.2) . Elevated concentrations in Old Tampa Bay and Lower Tampa Bay were primarily due to Pyrodinium bahamense and Karenia brevis blooms, respectively. Hillsborough Bay also showed elevated concentrations during two months in 2018 - the fall event coincided with blooms of the nonharmful alga, Tripos hircus.

Figure 2.2: Chlorophyll-a monthly averages from 1975-2018 for the four bay segments. The monthly averages for 2019 are shown in red.

2.3 Tampa Bay Seagrass Recovery

Tampa Bay’s total seagrass coverage continues to remain above the recovery goal, though a slight decrease in acreage was observed from 2016 to 2018. The baywide coverage was estimated to be 40,618 acres as of 2018 (Figure 2.3). As in 2016, total seagrass coverage remains above both the baywide target (38,000 acres) and the total estimated historic seagrass coverage of the 1950s (40,420 acres). The next SWFWMD seagrass coverage estimates will be developed from aerial photographs acquired over the winter 2019-20 period, following the extensive red tide event that plagued the region throughout 2018 (note: the 2018 coverage estimate for Tampa Bay was developed prior to the red tide affecting the Bay). More information on the Bay’s seagrass recovery utilizing transect monitoring data can also be found in TBEP Technical Publication #08-16.

Figure 2.3: Historic seagrass acreage estimates for Tampa Bay from 1950-2018